Wind turbines normally comprise one or more energy generating units, each energy generating unit comprising a hub carrying one or more wind turbine blades. The wind acts on the wind turbine blades, thereby causing the hub to rotate. The rotational movements of the hub are transferred to a generator, either via a gear arrangement or directly, in the case that the wind turbine is of a so-called direct drive type. In the generator, electrical energy is generated, which may be supplied to a power grid.
Some wind turbines are provided with two or more energy generating units in order to increase the total power produced by the wind turbine, without having to provide the wind turbine with one very large, and therefore heavy, energy generating unit. Such wind turbines are sometimes referred to as ‘multirotor wind turbines’.
When erecting or dismantling wind turbines, it is normally necessary to use external crane equipment for erecting the tower and lifting one or more nacelles, hubs, wind turbine blades, etc. to the top of the tower. As the size of wind turbines increases, the size of the crane necessary for performing these operations also increases. It may be very difficult and expensive to rent this kind of crane equipment, and to transfer it to and from the operating site of the wind turbine. In some cases, the costs of renting the crane equipment may even exceed the costs of producing the wind turbine. It is therefore very desirable to avoid the use of external crane equipment for erecting and dismantling wind turbines.
GB 2 443 886 A discloses a wind turbine arrangement comprising a tower and at least two arms projecting outwards therefrom. A wind turbine comprising a rotor and a nacelle is attached to an end of each arm. The nacelles and rotors may be lifted or removed individually by winches mounted temporarily or permanently to the wind turbine structure.
EP 1 476 659 B1 discloses a wind power installation comprising a tower and at least one rotor unit mounted on a support being supported on the tower by a rotary bearing. The support may be rotated into a position in which one of the rotor units is arranged at a lowest possible point, and the rotor unit can be lowered from the support by means of a lowering cable device.